How rebreathing anaesthetic systems control PaCO2: studies with a mechanical and a mathematical model

Results from a proposed equation for rebreathing systems, (formula: see text), were compared with results from a mechanical model "lung" ventilated either with a Bain Breathing Circuit, or a circle system (Eger-Ethans type A) without soda lime. When values for carbon dioxide excretion (VCO2), dead spacetidal volume ratio (VD/VT), minute volume ventilation (VE), and fresh gas flow (VF) were varied over a wide range, the model and the equation yielded identical values of PaCO2. When VCO2 = 2.25 ml/kg, VD/VT = 0.5, and VE = 140 ml/kg, then fresh gas flows (VF) with both the equation and the model produced values of PaCO2 which were very close to those found by Bain and Spoerel in anaesthetized, artificially ventilated humans. It is concluded that the equation is an accurate mathematical representation of how rebreathing anaesthetic systems control PaCO2. It is expected that the equation will be useful in the clinical application of rebreathing anaesthetic systems, allowing the selection of minute volumes and fresh gas flows which will yield predictable PaCO2 values.